A conventional vehicle speed control device conducts cruise control (constant speed control) that maintains a travel speed of a vehicle to a set travel speed.
For example, in JP 8-192657A, a desired opening position of a throttle valve for the cruise control is changed to a given opening position (for example, a fully closed position).
Also, in JP 6-64461A (U.S. Pat. No. 5,392,215), a travel resistance of the vehicle is detected based on an engine output torque and acceleration, an optimum control gain is determined according to a travel resistance, and a throttle valve is controlled with a desired throttle opening position based on the control gain.
In the above vehicle speed control devices, when accelerator operation is conducted by a driver under a cruise control, and a requested acceleration that is requested by the accelerator operation exceeds a control acceleration made by the cruise control (acceleration override state), the accelerator operation is prioritized. That is, the cruise control is interrupted during the acceleration override.
The cruise control is restarted or resumed at a time when the acceleration override state is canceled, and the vehicle speed is reduced to the set speed. This speed reduction will cause uncomfortable feeling to the driver. More specially, the speed reduction becomes excessive under the condition where the travel resistance is large with the result that the vehicle speed becomes lower than the set speed. When acceleration is conducted in order to increase the lowered vehicle speed to the set speed from that state, hunting (vibration) occurs in the vehicle speed. On the contrary, the speed reduction will be insufficient under the condition where the travel resistance is small. As a result, the travel speed remains to exceed the set speed. When the travel speed is reduced to the set speed from that state, hunting also occurs in the vehicle speed.
In JP 8-192657A, the travel resistance in the travel of the vehicle is not taken into consideration. In JP 6-64461A (U.S. Pat. No. 5,392,215), a speed reduction control at the time of canceling the acceleration override state is not taken into consideration. Therefore, the above hunting is caused.